It has long been recognized that the data throughput of a digital communication system may be limited by noise, interference, and at least in the wireless context, by the physical channel effects referred to as fading. To counter these effects, it is common practice to send data in coded form, such that redundant information in the code makes decoding possible even in the presence of some corrupted information. Certain of these codes are designed such that the receiver can check the detected and decoded data sequence using an operation referred to as a Cyclic Redundancy Check (CRC). If the CRC succeeds, the data sequence is deemed to have been received successfully.
In the context of packetized communications, there are known procedures according to which the receiver will request the retransmission of a corrupted packet. In at least some such procedures, information from both the original packet and the retransmitted packet is used for decoding.
In the context of wireless CDMA communication, a certain class of protocols for requesting retransmission of corrupted packets is referred to as Hybrid ARQ (Automatic Repeat Request), or HARQ. In accordance with the known techniques of HARQ, the receiver checks, exemplarily by executing a CRC, for successful decoding of the received data sequence. The receiver sends the transmitter a positive acknowledgement signal, referred to as ACK, in the event of sucessful decoding. Additionally or alternatively, the receiver sends a negative acknowledgement signal, referred to as NACK, in the event of unsuccessful decoding. The NACK, or alternatively the absence of an ACK at a designated time, prompts the transmitter to retransmit the corrupted packet.
Various alternative protocols are included in the broad category of HARQ techniques. One such protocol is Incremental Redundancy (IR). According to the IR protocol, each packetized sequence of bits to be transmitted is referred to as a “subpacket.” If the first subpacket fails the CRC check, the receiver requests retransmission. The retransmitted subpacket is a new subpacket containing the same message information as the first subpacket, but also containing a greater number of redundant bits as an aid for decoding. Generally, at least one further subpacket is available for retransmission, containing even more redundant bits than the previous subpacket.
In the practice of the IR protocol at least on the reverse link of a CDMA system as described, e.g., in the CDMA2000 RevD standard, each subpacket is preceded or accompanied by a control signal which identifies the format of the subpacket. Format information is communicated in the form of an index, which if received correctly identifies a particular selection from a multiplicity of available formats. Knowledge of the format tells the receiver, among other things, what level of redundancy to expect in the particular subpacket that is to be received.
It will be appreciated that in order for the subpacket to be decoded correctly, it is important for the receiver to receive the format index correctly. One of the challenges in at least the field of CDMA has been to increase the reliability with which such control information is received. In particular, it would be advantageous to increase such reliability without allocating additional power to the control messages at the expense of system capacity.